tipc: discard MSG_CRYPTO msgs when key_exchange_enabled is not set

When key_exchange is disabled, there is no reason to accept MSG_CRYPTO
msgs if it doesn't send MSG_CRYPTO msgs.

Signed-off-by: Xin Long <[e-mail]>
Acked-by: Jon Maloy <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/tipc/link.c

···
 		return false;
 #ifdef CONFIG_TIPC_CRYPTO
 	case MSG_CRYPTO:
-		if (TIPC_SKB_CB(skb)->decrypted) {
+		if (sysctl_tipc_key_exchange_enabled &&
+		    TIPC_SKB_CB(skb)->decrypted) {
 			tipc_crypto_msg_rcv(l->net, skb);
 			return true;
 		}